Secondary air supply systems are disclosed in US 2003/0061805 A1 (JP-A-2003-83048) and U.S. Pat. No. 6,918,245 B2 (JP-A-2004-11585). Specifically, secondary air is supplied to the upstream of a catalyst in an exhaust passage of an internal combustion engine for purifying exhaust gas, thereby accelerating purification (oxidation) of HC, CO in exhaust gas and warming the catalyst using reaction heat thereof in the secondary air supply systems.
When the secondary air supply systems cause a disorder, emission becomes degraded. Accordingly, a pressure sensor is provided to the secondary air supply pipe, so that an abnormality diagnosis is performed in accordance with pressure detected using the pressure sensor in the above secondary air supply systems.
As time elapses, the air pump discharging secondary air increases in temperature due to heat generated by a coil of a drive motor and heat of exhaust gas discharged from the engine. Pressure of exhaust gas is applied in the outlet of the secondary air supply passage. Therefore, as the pressure of the exhaust gas increases, a load of the air pump becomes high. As a result, a heat amount generated by the air pump increases, and increase in temperature of the air pump is apt to become large. As temperature of the air pump increases, resistance of the coil of the drive motor increases, consequently, the pumping performance of the air pump decreases. As a result, a flow amount (pressure) of secondary air may decrease. In this condition, when the abnormality diagnosis of the secondary air supply system is performed by detecting the amount (pressure) of secondary air, diagnosis of normality and abnormality of the secondary air supply system may be erroneously performed due to decrease in pumping performance of the air pump caused by increase in temperature of the air pump.